As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercially and widely used.
When a secondary battery is used as a power source for mobile phones or laptop computers, it is necessary for the secondary battery to stably provide uniform output. When a secondary battery is used as a power source for power tools, such as electric-powered drills, on the other hand, it is necessary for the secondary battery to provide instantaneously high output while the secondary battery is stable against external physical impacts, such as vibration or dropping.
In this connection, the structure of a conventional cylindrical secondary battery is shown in FIG. 1. Generally, a secondary battery 10 includes a cylindrical container 20, a jelly-roll type electrode assembly 30 mounted in the container 20, and a cap assembly 40 coupled to the upper part of the container 20.
The electrode assembly 30 is constructed in a structure in which a cathode 31 and an anode 32 are wound in a jelly-roll shape while a separator 33 is interposed between the cathode 31 and the anode 32. To the cathode 31 is attached a cathode tap 34, which is connected to the cap assembly 40. To the anode 32 is attached an anode tap (not shown), which is connected to the lower end of the container 20.
The cap assembly 40 includes a top cap 41 constituting a cathode terminal, a positive temperature coefficient (PTC) element 42 for intercepting electric current through the increase of battery resistance when the interior temperature of the battery is increased, a vent 43 for intercepting electric current and/or discharge gas when the interior pressure of the battery is increased, a gasket 44 for electrically isolating the vent 43 from a cap plate 45 excluding a specific portion and sealing the interior of the battery, and the cap plate 45 connected to the cathode tap 34, which is attached to the cathode 31. The cap assembly 40 is constructed in a structure in which the top cap 41, the PTC element 42, the vent 43, the gasket 44, and the cap plate 45 are sequentially stacked one on another.
However, it has been proven that it is difficult for the secondary battery with the above-stated construction to provide instantaneously high output, and the resistance of the contact surfaces of the secondary battery is increased when external impacts, such as vibration, is applied to the secondary battery, whereby it is difficult for the secondary battery to provide uniform output. Specifically, the PTC element 42 has an electrical resistance of approximately 7 to 32 mΩ at room temperature, and the resistance of the PTC element 42 is sharply increased when the temperature is increased. Consequently, the PTC element 42 may serve as a factor to stop the provision of instantaneously high output. For this reason, a structure excluding the PTC element 42 is needed.
Nevertheless, the secondary battery with the above-stated construction has a problem in that, when external impacts, such as vibration, are applied to the secondary battery, the resistance at the contact surfaces between the top cap 41, the PTC element 42, the vent 43, and the cap plate 45 is greatly changed, and therefore, the secondary battery cannot provide uniform output. For example, the resistance at the contact surfaces between the top cap 41 and the PTC element 42 or the vent 43 is increased by approximately 20 to 30 mΩ.
The increase of the interior resistance induces generation of heat from a high-current secondary battery used as a power source for power tools. As a result, the safety of the battery may be lowered, and therefore, the performance of the battery may be decreased.
In this connection, Japanese Unexamined Patent Publication No. 2003-187773 discloses a technology for bending the end of a vent such that the outer circumferential surface of a top cap can be surrounded by the bent end of the vent and fixing the bent end of the vent to the top cap by welding so as to prevent the contact resistance between the vent and the top cap from being increased due to extreme vibrations or impacts applied to a battery and the degradation of a gasket resulting from the long-term use of the battery.
Even though there is a difference in terms of purpose and effect, a technology for surrounding the top cap using a member, such as a vent, is disclosed in Japanese Unexamined Patent Publication No. 2004-152707, and a technology for attaching the end of a battery container and a seam part to each other by laser welding is disclosed in Japanese Unexamined Patent Publication No. 2003-051294.
The technology for surrounding the outer circumferential surface of the top cap with the end of the vent has an advantage in that the increase of the contact resistance due to the vibrations or impacts is restrained, as compared to the structure in which the vent and the top cap are in surface contact with each other, and therefore, a battery adopting this technology can be used as a high-current power source. However, it has been proven that, when the battery adopting this technology is used in devices that induce great vibrations, such as power tools, the contact surfaces between the vent and the top cap are instantaneously spaced apart from each other in spite of the above-described structure, and an electrolyte leaks out of the battery through the resulting gap. Furthermore, it has been proven that the fixed state of the top cap fixed to the vent is not stable, in spite of the bent structure of the end of the vent, with the result that the relative movement between the top cap and the vent is caused, and therefore, during the assembly process, specifically, when a current intercepting member is attached to the lower end of the vent by welding or the vent and the top cap are attached to each other by welding, the welding operations are not performed at predetermined positions (correct positions), whereby the defectiveness of the battery is caused.
On the other hand, it has been proven that, although the technology for attaching the bent end of the vent to the top cap by welding provides an excellent coupling force, the welded portions are exposed to the outside of the battery, and the exposed portions are degraded due to the long-term use of the battery, whereby the coupling force is considerably lowered.